1. Field of the Invention
The present invention relates to an apparatus for cooling a tubular polymer film material which is being extruded, and more particularly, pertains to an apparatus for extruding tubular thermoplastic films including two, or tandem spaced air rings for controlling the cooling rate and production speeds of the thermoplastic films during extrusion thereof. Moreover, the invention also relates to a method for the cooling of an extruded tubular thermoplastic film material through the utilization of a tandem air ring cooling structure.
In order to produce a continuous tubular film web from a molten polymer material upon extrusion of the material through the orifice of an annular extrusion die, the formed tubular film or bubble is cooled in order to solidify or essentially crystallize the molten polymer material and, depending upon the process, the material is cooled by external and/or internal cooling of the formed tubular film bubble.
Among various types of extrusion apparatus and processes currently employed in the technology, cooling of the extruded tubular polymer is effectuated by contacting the external surface of the formed tubular film; for example, which may be constituted of a linear or high-density ethylene polymer, with a cooling air flow being directed about the external peripheral surface and in the feed or machine direction of the advancing bubble or tubular film. This is implemented by positioning an annular air ring directly downstream of the discharge or orifice of the extrusion die, preferably by positioning the air ring on the extrusion die, so as to immediately cool the extruded molten polymer upon exiting from the extrusion die.
In order to enhance upon the rate and quality of the cooling of the extruded thermoplastic tubular film web, at least two or more air rings have been positioned in a spaced relationship to each other in the longitudinal direction of travel of the tubular film such that the flow of cooling air from the first air ring which is proximate the orifice of the extrusion die, is supplemented by a flow of cooling air from a second or tandem air ring positioned to extend about the tubular film web downstream of the first air ring.
Although the foregoing employment of tandem air rings has proven itself to be advantageous in comparison with the use of only a single cooling air ring, apparatus and methods for cooling the bubble or tubular film web through the intermediary of a plurality of spaced or tandem air rings have, in many instances, been somewhat inadequate in imparting a uniformity to the gauge of the plastic film tube or web, nor permitted for the implementation of sufficiently high production speeds, particularly for linear low-density polyethylene resins possessing low melt tension, so as to produce tubular film products embued with an accurate degree of gauge thickness and uniform quality.
The present invention is intended to improve upon prior art apparatus and methods for the cooling of extruded tubular thermoplastic film web materials currently utilized in the technology through the utilization of tandem air rings, by providing an enhanced control over the cooling air flow imparted to the external peripheral surface of the extruded tubular film material or bubble through the intermediary of an annular shroud member which is interposed between the two tandem air rings so as to enable a controllable air flow to be induced downstream of the first air ring and diverted prior to reaching the tandem or second air ring; while in addition thereto, the spacing or distance between the air rings may be varied so as to additionally enhance the degree or rate of cooling of the airflows impinging against the film web.
Various publications directed to methods and apparatus for the utilization of multiple or tandem air rings for cooling extruded tubular thermoplastic film materials are currently known in the technology.
Jones, et al. U.S. Pat. No. 4,330,501 discloses a method and apparatus for cooling a film bubble which is constituted of a thermal plastic polymer, wherein a first air ring positioned immediately downstream of and extends about the orifice of a tubular extrusion die causes an air flow to pass along the outer peripheral surface of the extruded air bubble, and with the air flow from the first air ring being supplemented by an additional cooling air flow from a second or tandem air ring positioned to extend about the film bubble at the location downstream of the first air ring. The air flow from the first air ring is passed along the surface and admixed with the air flow from the tandem air ring in view of an encompassing barrier or shroud extending between the air rings which forces the entire air flow from the first air ring to be passed along the surface of the extruded plastic tube or bubble past the tandem air ring.
Blakeslee, III et al. discloses a tandem air ring structure for cooling an extruded thermoplastic tubular film material, in which the presence of an annular barrier extending between the first and second air rings forces the flow of cooling air from the first air ring to be passed along the surface of the extruded tubular film and admixed with the airflow from the tandem or second air ring.
Herrington U.S. Pat. No. 3,959,425 discloses a method for extruding and cooling tubular thermoplastic film, in which the tubular film is cooled subsequent to exiting from an extrusion die through the utilization of a plurality of superimposed air rings, in which the air flow from the collective air rings is combined into a single flow longitudinally extending along the external peripheral surface of the extruded tubular film material.
Although all of the foregoing multiple air ring structures for cooling the external surface of a tubular thermoplastic film material, among others employed in the technology, are generally satisfactory in operation, an inadequacy is encountered in controlling the expansion and quality of the extruded film material, inasmuch as the quantity of the cooling air flowing between the first and second air rings cannot be controlled in a precise manner.